Stimuli that increase membrane permeability to K+ produce
A stronger stimulus produces a larger hyperpolarization.
Stimuli that increase membrane permeability to Na+ produce
A stronger stimulus produces a larger depolarization.
Some channels are voltage-gated:
a depolarization causes more Na+ channels to open.
A depolarization that reaches the threshold (-55 mV)
triggers positive feedback and
causes a massive voltage change called action potential (30 mV).